1 Introduction

The Early Paleozoic ocean-continent transition in the Qinling-Qilian orogenic belt (Xu et al. 2008; Dong et al. 2021) preserves important information about the Proto-Tethys ocean (Fig. 1a). However, the tectonic implication of the Early Paleozoic igneous rocks and sediment in the Qinling-Qilian junction zone (Feng et al. 2003; Pei et al. 2009; Dong et al. 2021) remains controversial. Xu et al. (2008) summarized the Early Paleozoic magmatic rocks, metamorphism, and sedimentary strata in this area; they proposed that the North Qilian Ocean and North Qinling Ocean were connected until the Ordovician. The metamorphic basalts with N-MORB affinity in the Guanzizhen and Wushan ophiolites of the North Qinling orogenic belt have similar features to the Chisha N-MORB type basalts of the North Qilian Mountain (Fig. 1b). However, the tectonic evolution and deep geodynamics of the Late Ordovician-Late Silurian in this junction zone remains poorly understood. The Hongtubu metamorphic basalt (443 ± 2 Ma) in the eastern section of the North Qilian orogenic belt indicates an Ordovician back-arc basin in the area (Xu et al. 2008) (Fig. 1b). The Chenjiahe intermediate-acidic rocks display island arc affinity (Fig. 1b). Li et al. (2017) reported that the Early Ordovician (436 ± 3.8 Ma) Guanshan gabbro displays transitional geochemical characteristics between MORB and IAB, indicating back-arc setting (Fig. 1b). These basic rocks have similar ages with the granite-diorite (Qin et al. 2022), indicating coupled melting of crust and mantle in extensional setting. The genetic links of these rocks are of great significance for exploring the geodynamic process and crust-mantle evolution in this area.

Fig. 1
figure 1

a Geological map of China. b Geological map in the North Qilian-North Qinling orogenic belt (modified after Xu 2008). 1: Carboniferous-Cenozoic; 2: Devonian; 3: Silurian; 4: Chenjiahe Group (meta-acid volcanic rocks); 5: Caotangou Group (meta-volcanic rocks); 6: Hongtubu Group (meta-basic volcanic rock); 7: Huluhe Group (meta-clastic rocks); 8: Xieyuguan Group; 9: Danfeng Group; 10: Muqitan Formation; 11: Kuanping Group; 12: Longshan Group; 13: Qinling Group; 14: Mesoproterozoic gneissic monzonitic granite; 15: Caledonian quartz diorite pluton; 16: Indosinian intermediate-acidic rock mass; 17: Mesozoic-Cenozoic (Ordos Basin sedimentary strata); 18: Ordovician (Ordos Basin sedimentary strata); 19: ultrabasic rock; 20: fault. c Geological map of the Longxian area (modified after Weng 2012)

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This paper focuses on a suit of Late Silurian basalts in the Longxian area, southwestern margin of the Ordos Basin (Fig. 1b, c). Through the results of zircon LA-ICP-MS U-Pb dating, major and trace elements and Sr-Nd isotope, we discuss the potential magma source, partial melting mechanism and tectonic implication. The results indicate that the Angou basalts were formed in a back-arc setting, which can place important constraints on the mantle source at the junction zone of the Qinling-Qilian orogenic belt.

2 Regional geological background and field geological characteristics

The Qinling-Qilian junction zone is composed of the western part of the North Qinling and the eastern region of the North Qilian, which is divided into three tectonic units (Mao et al. 2017) (Fig. 1a, b): the Liziyuan subduction complex belt (Shangdan Suture), Qinling arc metamorphic-magmatic complex belt and Qingshui-Zhangjiachuan back-arc complex belt. The Early Paleozoic ophiolite in the Liziyuan subduction complex belt represents the remnants of the Cambrian Proto-Tethys oceanic crust (Pei et al. 2004, 2009; Li et al. 2007; Ding et al. 2008); the Caotangou volcanic-sedimentary rocks and Early Paleozoic granitoid and diorites are parts of Qinling arc metamorphic-magmatic complex belt, indicating an arc environment related to oceanic subduction (Mao et al. 2017). The Qingshui-Zhangjiachuan back-arc complex belt consisted of Huluhe Group clastic rocks, Chenjiahe volcanic-sedimentary rocks and Hongtubu metabasalts (Fig. 1b). Detrital zircon U-Pb dating indicates that the Huluhe Group was formed in the Early Silurian (447–433 Ma, Ren et al. 2021). The intermediate-acid volcanic rocks in the Chenjiahe have zircon U-Pb age of 448–447 Ma and display typical island-arc affinity (He et al. 2007a). Metabasalts in the Hongtubu have zircon U-Pb age of 484–386 Ma and were formed in the back-arc setting (Hu 2005; He et al. 2007a, b).

The spatial-temporal relationship of igneous rocks suggests a typical Early Paleozoic active continental margin with trench-island arc system in the Qinling-Qilian junction zone, which resulted from multi-stage subduction and collision of the Proto-Tethys (Dong et al. 2021; Fu et al. 2019; Pei et al. 2009; Xu et al. 2008; Zhang et al. 2006; Qin et al. 2022) (Fig. 1a).

The Angou basalt is located in the southwest margin of the Ordos Basin, along the northeastern direction of the Qingshui-Zhangjiachuan back-arc complex belt, which can place significant constraints on the Late Ordovician-Late Silurian back-arc setting and tectonic evolution and geodynamic process of the Qinling-Qilian junction zone (Fig. 1b, c).

The Angou basalt mainly occurred in the Baicaoshan, Angou and Erxianzi, 12 km southwest of Longxian County (Fig. 1c), located in the southern part of the western margin of Ordos Basin. The basalts are 1.5 km long and 0.5 km wide in the Baicaoshan-Anguo area. Some deformation structures can be observed in the margin section. The basalt is mainly composed of plagioclase and clino-pyroxene (Fig. 2). Plagioclase has euhedral crystals with slight alteration, clino-pyroxene is slightly larger and filled between plagioclase particles, and some olivine grains with irregular cracks can be found (Fig. 2).

Fig. 2
figure 2

Outcrop photos and microscopic pictures for the Angou basalt from the junction zone between the North Qinling and Qilian orogenic belt. a Basalt sampling point. b Vesicular structure and a large number of amygdales developed in the basalts. c Fresh surface of basalt sample. d Olivine mineral photos under microscope. e Photographs of clinopyroxene and plagioclase minerals under microscope. f Plagioclase mineral photos under microscope. OI: olivine; Cpx: clinopyroxene; PI: plagioclase

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3 Results

3.1 Zircon LA-ICP-MS U-Pb chronology and trace element characteristics

The sample from the Angou basalt was selected for zircon LA-ICP MS U-Pb dating. The analysis results are listed in Table 1.

Table 1 Results of LA-ICP-MS U-Pb dating of zircons from the Angou basalt
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Most zircons (A-2X) display euhedral or subhedral columnar texture, with significant changes in size (60–200 µm). The CL image has a wide and unclosed ring, indicating the characteristics of primary zircon in mafic rocks (Fig. 3). Fourteen out of 22 clustering spots have Th/U ratios of 0.16 to 0.85 (Table 1), indicating the magmatic genesis. These spots display 206Pb/238U ages ranging from 419 to 438 Ma (Fig. 4), yielding a weighted mean age of 428 ± 5 Ma (MSWD = 0.30, n = 14) (Fig. 4).

Fig. 3
figure 3

Cathodoluminescence (CL) images for the Angou basalt from the junction zone between the North Qinling and Qilian orogenic belt

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Fig. 4
figure 4

LA-ICP-MS U-Pb concordia diagrams for the Angou basalt from the junction zone between the North Qinling and Qilian orogenic belt

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Some older basalts and related rocks have been reported in the junction zone of the Qinling-Qilian orogenic belt, i.e., Hongtubu basalt (443.4 ± 1.7 Ma) (He et al. 2007a) and Chenjiahe intermediate acid volcanic rocks (447 ± 8.5 Ma) (He et al. 2007a), suggesting extensive mantle melting events in the southwestern Ordos Basin.

3.2 Major and trace elements

The geochemical analysis results of the Angou basalt are shown in Table 2. Due to the slight alteration, these rocks have relatively high loss on ignition of 2.55 wt% to 3.39 wt%. In this paper, the major elements were corrected for loss on ignition.

Table 2 Major element and trace element compositions of Angou basalt samples
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The Angou basalt has SiO2 = 49.52–50.39 (wt%), Al2O3 = 13.53–14.79 (wt%), with a low total alkali content (Na2O + K2O) of 4.53–5.43 (wt%) (Fig. 5a), K2O/Na2O ratios of 0.12–0.20, indicating a typical Na-rich basalt. The TFe2O3 = 12.68–15.46 (wt%), MgO = 6.93–7.98 (wt%), Mg# = 47–50, which is significantly lower than the primitive basaltic magma (Mg# = 68–75, Frey et al. 1978), indicating an evolved basaltic magma. All samples have tholeiitic basalt affinity with high FeO*/MgO ratios (Fig. 5b).

Fig. 5
figure 5

a TAS discriminant diagram (modified after Le and Streckeisen 1991); b SiO2-(FeO*/MgO) diagram (modified after Le and Streckeisen 1991) for the Angou basalt from the junction zone between the North Qinling and Qilian orogenic belt

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There is no obvious Eu anomaly (δEu = 0.96–1.32, 1.08), indicating no significant crystallization of plagioclase (Fig. 6a). These features are identical to the enriched mid-ocean ridge basalt (E-MORB). The large ion lithophile elements (LILE) K, Rb, Sr and Ba are relatively enriched, indicating an origin from enriched mantle source areas (Fig. 6a). The high field strength elements (HFSE) Nb, Ta and Ti are similar to E-MORB and reflect the characteristics of intraplate basalt (Fig. 6a). Zr exhibits losses relative to N-MORB, which may be related to the source region. The relative enrichment of incompatible elements (Fig. 6a) reflects that the parent magma may have originated from the enriched lithospheric mantle. The basalts have relatively low total rare earth element contents (∑REE = 67.95–108.31 ppm) and display slightly enriched LREE patterns (Fig. 6b), LREE/HREE ratio of 1.48–1.73, (La/Yb)N ratio of 4.66–5.53.

Fig. 6
figure 6

a Primitive mantle-normalized trace element spider diagram (primitive mantle values are from Mcdonough et al. 1992). b Chondrite-normalized REE patterns (Chondrite values are from Boynton 1984) for the Angou basalt from the junction zone between the North Qinling and Qilian orogenic belt. The date of N-MORB, E-MORB and OIB is from Sun and McDonough, (1989), trace element of the Pm306 is from Li et al. (2018); and trace element of the 16CJH is from Fu et al. (2019)

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3.3 Sr-Nd isotopic composition

The Sr-Nd isotopic composition analysis results of basalts are shown in Table 3. The Angou basalts have 87Sr/86Sr = 0.7147–0.7157, 143Nd/144Nd = 0.5122–0.5123, εNd (t) = − 5.55 to − 3.40, t2DM = 2.830–2.927 Ga. As shown in the (87Sr/86Sr)i vs εNd (t) diagram, they display enriched Sr-Nd isotopic compositions, which plotted within the range of EMII (Fig. 7). Compared with the Sr-Nd isotopic compositions of the Early Paleozoic basalts in the Qinling-Qilian junction zone, these samples also show the more evolved Sr-Nd isotopic compositions, suggesting an enriched mantle source.

Table 3 Rb-Sr and Sm-Nd isotopic data of Angou basalt
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Fig. 7
figure 7

(87Sr/86Sr)i vs. εNd (t) plot for the Angou basalt from the junction zone between the North Qinling and Qilian orogenic belt. (modified after Qin et al. 2022). Sr-Nd isotopic composition of the Cambrian mafic rocks from Fu et al. (2019); Sr-Nd isotopic composition of the Early Ordovician mafic rocks from Hu et al. (2005); Sr-Nd isotopic composition of the late Ordovician mafic rocks from He et al. (2007a). Data of the MORB, oceanic arc basalts in the Shangdan suture, metabasites from the North Qinling and gneisses from the North Qinling are from Wang et al. (2014b) and references therein

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4 Discussion

4.1 Evaluation of post-magmatic processes

Loss on ignition (LOI) is proportional to the alteration degree. Considering that Angou basalt has a high loss on ignition (2.55–3.39), it is necessary to evaluate the alteration degree. The Angou basalt has igneous texture. The plagioclase and clinopyroxene display slightly alteration. However, their consistent trace and REE contents indicate insignificant alteration. Zr remains stable in most of the alteration and metamorphism (Polat, 2002). There is a good correlation between the elements (Fig. 8), indicating slight alteration. In conclusion, the Angou basalt has slight alteration.

Fig. 8
figure 8

The relationship of Zr with other elements for the Angou basalt from the junction zone between the North Qinling and Qilian orogenic belt. a Ratio diagram of Zr content to Ba content. b Ratio diagram of Zr content to U content. c Ratio diagram of Zr element content to Sr content. d Ratio diagram of Zr content to Yb content. e Ratio diagram of Zr content to Th content. f Ratio diagram of Zr content to magnesium oxide. g Ratio diagram of Zr content to total alkali content. h Ratio diagram of Zr content to Nb content. i Ratio diagram of Zr content to Nd content

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Mantle-derived mafic magmas may be contaminated by crustal material during their ascent process. It is necessary to evaluate the potential impact of crustal contamination. The Angou basalt is enriched in LILEs and LREE (Table 2) and has E-MORB-type trace element spider diagram and REE pattern (Fig. 6), which significantly differs from the N-MORB type basalt (Fig. 6). The continental crust has enriched SiO2, LILEs, LREE and evolved Sr-Nd isotopic compositions (Gao et al. 1998; Liu et al. 2004). The contaminated basalts may be enriched in LILES and LREE and depleted in HFSEs. The continental crust has significantly higher ratios of Th/Nb and Nb/U (Th/Nb = 0.65, Nb/U = 9.17, Gao et al. 1998; Th/Nb = 0.7, Nb/U = 6.15, Rudnick et al. 2003) than the MORB (Th/Nb = 0.09–0.1; Nb/U = 33.33–36.67, Klein et al. 2003) and OIB (Th/Nb = 0.135, Nb/U = 28.57, Taylor et al. 2002). The Angou basalts have low Th/Nb (0.08–0.09) and high Nb/U (29.04–36.14), which significantly differ from the continental crust and are close to the MORB and OIB range. There is an absence of negative Ti anomaly also against significant continental contamination (Fig. 6). In summary, we suggest that the Angou basalt has insignificant crustal contamination. The enriched LILEs and LREE pattern and evolved Sr-Nd isotopic compositions may have been inherited from the mantle source.

The Angou basalt has consistent MgO and SiO2 contents (Fig. 9a), indicating a homogeneous parent magma. There is weak negative correlation among TiO2, Fe2O3T and MgO, and the low Ti “anomaly” may be inherited from a Ti-depleted source (Fig. 9). The positive correlation among Ni, Al2O3, Cr and CaO with MgO indicates fractional crystallization of olivine, pyroxene and plagioclase (Fig. 9). The Angou basalts have lower Mg# (47.25–50.27) than the primitive basalt (Mg# = 68–75, Frey et al. 1978). It also has lower solidification index (SI) of 26.5–33, (SI) % = 100 ×w(MgO)/[w(MgO) + w(FeO) + w(Fe2O3) + w(Na2O) + w(K2O)] than the primitive basalt (SI around 40). The positive correlation between MgO and Ni indicates the forsterite fraction, while a positive correlation between CaO and MgO (Fig. 10) indicates the fraction of Ca-rich pyroxene (such as augite). The weak positive correlation between CaO and Al2O3 (Fig. 10) indicates the fraction of anorthite, which is consistent with the weak Eu anomalies (Fig. 6a). There is a negative correlation between Al2O3 and Na2O (Fig. 10) and negative correlation between MgO and Na2O (Fig. 9f); these features indicate that albite is not the fraction phase.

Fig. 9
figure 9

Major elements of Harker diagrams for the Angou basalt from the junction zone between the North Qinling and Qilian orogenic belt (modified after Irvine and Baragar, 1971)

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Fig. 10
figure 10

Bivariate plots of major element oxides showing fractional crystallization trends for the Angou basalt from the junction zone between the North Qinling and Qilian orogenic belt. a Ratio of magnesium oxide to total iron. b Ratio of magnesium oxide to nickel. c Ratio of calcium oxide to magnesium oxide. d Ratio of alumina to calcium oxide. e Ratio of alumina to sodium oxide

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Although the Angou basalt is enriched in Th and U (relative to N-MORB), it has consistent εNd (t) and U/Nb ratios (Fig. 11a and b), indicating that the "crustal source" feature may be inherited from the enriched mantle source rather than continental contamination. The basalt has similar Nb/Ta (16.4–17.3) and Lu/Yb (0.14–0.15) ratios to the mantle (Nb/Ta = 17.6, Lu/Yb = 0.14–0.15, Sun and McDonough 1989; Weyer et al. 2002) (Fig. 11c, d). Their consistent evolved Sr-Nd isotopic composition (Fig. 7) also indicates an enriched mantle source rather than crustal contamination.

Fig. 11
figure 11

Diagram of a SiO2-εNd (t), b SiO2 vs. U/Nb, c SiO2 vs. Nb/Ta (modified after Weyer et al.2002), d SiO2 vs. Lu/Yb (modified after Sun and McDonough 1989) for the Angou basalt from the junction zone between the North Qinling and Qilian orogenic belt

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4.2 Decompression melting of enriched mantle source

Several lines of evidence indicate that the Angou basalt was derived from an enriched mantle source, i.e., enriched in LILE (K, Rb, Ba, Sr), which may be attributed to the enrichment of the lithospheric mantle (Downes 2001; Wang et al. 2014a; Liu et al. 2022). The high Ba/Th and Nb/Y ratios indicate that their mantle source was metasomatized by subduction-related fluid and melts (Fig. 12a, b). Possible metasomatic agents include: (1) continental crust-derived felsic melts; (2) oceanic slab-derived fluid; (3) fluid derived from sediments in the subduction channel (Rapp and Watson 1995; Hanyu et al. 2006); (4) fluid and melts derived from enriched mantle; (5) melts that derived from dehydration melting of subducted oceanic slab.

Fig. 12
figure 12

Diagram of a Th vs. Ba/Th (modified after Zhao et al. 2020), b Nb/Y vs. Rb/Y (modified after Zhao et al.2020) for the Angou basalt from the junction zone between the North Qinling and Qilian orogenic belt

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The Qinling-Qilian orogenic belt was affected by the northward subduction of the Shangdan Ocean from 530 to 430 Ma (Dong et al. 2021). Due to the absence of a continental collision event, the subducted continental crust- or delamination thickened crust-derived melts cannot be the lithospheric mantle metasomatic agent. The Sr-Nd isotopic composition of the Early Paleozoic basalts in the Qinling-Qilian junction zone has changed from depletion to enrichment (Hu 2005; He et al. 2007b; Fu et al. 2019) and has coupling relationship with the Shangdan Ocean subduction. Subducted oceanic slab-derived fluid has enriched LILEs and LREE and depleted HFSE (i.e., Nb, Ta and Ti) geochemical features (Zhao 2016); metasomatized mantle wedge melting products inherit these geochemical features, with high La/Nb > 1.4 (Rudnick 1995; Condie 1999). Although Angou basalt displays enriched LILEs, the La/Nb ratio is < 1.4 (0.93–1.05), and there are no negative HFSE anomalies. Dehydration of oceanic crust rocks showed that Pb has greater solubility and mobility than U, Rb than Sr and Nd than Sm in fluid (Kogiso et al. 1997). Therefore, subducted oceanic slab-derived fluid has high Rb/Sr, low U/Pb and Sm/Nd ratios. The metasomatized mantle wedge melting product has EM-1 isotope geochemical feature, which was high 87Sr/86Sr value, the lowest 143Nd/144Nd value and 206Pb/204Pb after long-term metasomatism. However, the Angou basalt has EM-II isotope geochemical feature (Fig. 7). The EMII-enriched mantle has the highest 87Sr/86Sr ratio, which was considered to result from mixing of continental sediments recycled with the subducted oceanic crust and mantle (Zindler and Hart 1986). The Sanchahe high Mg# diorite (459 Ma) was considered to be derived from hydrous melting of sediment-derived Si-rich melt metasomatized mantle wedge (Qin et al. 2022) in Qinling-Qilian junction. However, the terrigenous sediments have obvious Nb, Ta, Ti and Eu negative anomalies and low Ce/Pb values (Jackson et al. 2007); this is inconsistent with the Angou basalts. Intermediate-felsic intrusive rocks (440–430 Ma) with depleted zircon Hf isotopic compositions were considered to form in the circumstance of oceanic slab rollback and slab break-off and upwelling of asthenosphere in Qinling-Qilian junction (Qin et al. 2022). These indicate the lack of enriched mantle source metasomatic agent. The Angou basalt has Nb/Ta ratios of 16.45–17.28 and oceanic crust has high Nb/Ta ratios (DM = 15.5 ± 1; Rudnick et al. 2000; N-MORB = 17.7; Sun and McDonough 1989). Combined with the absence of negative HFSE anomalies and the enriched LREE patterns, the metasomatic agent may be oceanic crust-derived melts. However, if the metasomatic agent is only oceanic crust-derived melts, the Angou basalt will not have the over-enriched Sr isotope (0.7128–0.7140). Altered oceanic crust (AOC)-derived fluid has “excess” 87Sr/86Sr and high Sr concentrations (Staudigel et al. 1995; Hauff et al. 2003; Turner and Langmuir 2022). Global arc volcanoes derived from AOC-fluid metasomatized mantle tend to have Sr-Nd isotope decoupling (higher 87Sr/86Sr) and high Ba/Th ratios (> 400), and Sr-Nd isotopic decoupling degree is proportional to the Ba/Th (Zhang et al. 2024). The Qinling-Qilian junction Early Paleozoic basalts have normal Sr-Nd isotopic composition and low Ba/Th values (Table 4) of 17–232 (average = 99), but the Angou basalt has Sr-Nd isotope decoupling and high Ba/Th values (Table 4) of 260–1009 (average = 663), indicating that another metasomatic agent may be altered oceanic crust-derived fluid. Combined with La/Nb < 1.4 (0.93–1.05) and no obvious HFSE negative anomaly in Angou basalt, we suggest that the AOC-fluid metasomatism time and content are less.

Table 4 Ba/Th values of the Early Paleozoic basalts in the Qinling-Qilian junction zone
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The incompatible element ratios, such as Sm/Yb and Dy/Yb, can suggest the composition of the mantle and partial melting degree (Shaw 1970; Xu et al. 2005; Duggen et al. 2005). In the Sm/Yb-Sm diagram (Fig. 13a), the Angou basalt displays transitional features derived from spinel lherzolite and garnet lherzolite (1:1). In addition, the moderate Dy/Yb (1.82–1.98) ratio of the Angou basalt (Fig. 13b) also indicates the transition zone feature of the spinel garnet-stable domain (Deng et al. 2017).

Fig. 13
figure 13

Diagram of a Sm vs. Sm/Yb (modified after Qi 2021), b (K/1000Yb) vs. (Dy/Yb) (modified after Duggen et al. 2005) for the Angou basalt from the junction zone between the North Qinling and Qilian orogenic belt

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4.3 Changes from depleted to evolved mantle source in the junction zone of the Qinling-Qilian orogenic belt

Voluminous Late Ordovician-Late Silurian granites and associated basic volcanic rocks were in the Tianshui-Longshan area (Fig. 1b). Previous studies have suggested that these rocks are products of the Early Paleozoic subduction collision process of the Shangdan and Qilian Oceans (Pei et al. 2009; Wang et al. 2012; Xu et al. 2008; Dong et al. 2021; Qin et al. 2022). The Early Paleozoic basalts from the Hongtubu, Lajishan and Saozhoutan have island arc basalt and back-arc affinity, which were formed in back-arc setting (Hu 2005; He et al. 2007b; Dong et al. 2011a; Wei 2013; Li et al. 2017; Li et al. 2018; Fu et al. 2019). In the early evolution stage of the back-arc setting (immature back-arc basin), the subduction-related fluid and melts are more involved. In the late evolution stage back-arc basin (mature back-arc basin), the influence of subduction fluid and melts is gradually weakened, and BABB (back arc basin basalt; Hawkins et al. 1990) has the mid-ocean ridge basalt (MORB) geochemical feature (Klein et al. 1987). In the Ta-Hf/3-Th diagram, the Qinling-Qilian junction Early Paleozoic basalts plotted in the IAB and MORB region. With the continuous subduction of the Shangdan Ocean and the expansion of the back-arc basin, the IAB-type basalts gradually decreased and evolved into MORB-type basalts. The Late Silurian (428 ± 5 Ma) Angou basalt has typical EMORB-type trace element geochemical feature (Fig. 6), representing mature back-arc basin. Compared with the Hongtubu and Lajishan basalts (Li et al. 2018; Fu et al. 2019), the Angou basalt has younger ages (428 ± 5 Ma); it also has evolved Sr-Nd isotopic composition, indicating the subduction process of the Proto-Tethys Ocean since the Cambrian. The fluid and melts from the subduction zone continuously metasomatize the overlying mantle wedge. The mantle wedge shows a gradual evolution trend from depletion to evolved (Fig. 7). In the Hf/3-Th-Ta tectonic discrimination diagram (Fig. 14), the Angou basalt plotted in the intraplate tholeiite area, indicating that it is formed in an extensional setting. Combined with the coeval voluminous I-type granites, these igneous rock associations indicate that the mantle properties of the junction zone of the Qinling-Qilian moved to the evolved stage in the Late Silurian (~ 420 Ma). Under the extensional setting, the evolved mantle and lower crust undergo coupled partial melting. This melting process may occur in the mature island arc area or the extensional setting in the post-collision setting. Based on the above discussion, the Angou Late Silurian basalt can represent partial melting of the metasomatic enriched lithospheric mantle under the post-collision extensional setting, which is consistent with the formation age of the contemporaneous I-type granite (Ren et al. 2021; Qin et al. 2021), representing the crustal mantle interaction process in the matured island arc in the post-collision setting.

Fig. 14
figure 14

Th–Hf/3–Ta triangle diagram of basalt tectonic discrimination (modified after Wood 1980). A N-type MORB; B E-type MORB and tholeiitic within-plate basalts and differentiates; C alkaline within-plate basalts and differentiates; D destructive plate-margin basalts and differentiates. Date of Late Cambrian basalt from Fu et al., (2019); date of Early Ordovician basalt from Hu (2005); date of Late Ordovician basalt from He et al. (2007a, b) and Li (2008); date of Early Silurian basalt from Li et al. (2017)

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5 Conclusions

  1. (1)

    The Late Silurian (428 ± 5 Ma) Angou basalt in junction zone of the Qinling-Qilian orogenic belt displays high Na2O/K2O ratios, enriched LILE and LREE patterns, combined with its enriched Sr-Nd isotopic composition. This indicates that it originated from the decompressional melting metasomatized enriched mantle in extensional setting.

  2. (2)

    Compared with the Cambrian basalts in this region, the Angou basalts indicated a evolution trend from depletion to enrichment of the mantle source in the junction zone of the Qinling-Qilian orogenic belt. The formation of Angou basalts represents the partial melting of the enriched mantle in post-collision setting.